The primary objective of this proposal is to analyze the factors which allow for progressive growth of malignant Marek's disease virus (MDV)-induced tumors in chickens which have a demonstrable immune response to their tumors. Thus, the major goals will be to determine: a) what cellular and humoral immune responses are elicited in birds with primary or non-transplantable Marek's disease (MD) lymphomas as well as birds possessing a syngeneic transplantable MD lymphoma; b) what mechanisms may exists by which MD lymphomas escape from host immune responses; and c) if procedures might be developed which would repress mechanisms responsible for escape of MD lymphomas from host immunity. Information obtained from previous studies showing limited transplantability of most MDV-induced primary tumors passaged into syngeneic hosts will be utilized to further explore the host response to tumors. A highly malignant transplantable MD lymphoma, developed in one of our inbred lines, will also be used to study immunopathological properties associated with tumor invasion and metastasis. The availability of inbred lines of chickens in which we can do histocompatible cell transfers, the use of a syngeneic transplantable lymphoma and the development of vaccines highly effective against MD offer unique advantages for studying tumor and host factors affecting growth of herpesvirus induced lymphomas. Particular emphasis will be placed on studying ways to counteract subversion of host reactivity to progressively growing lymphomas.